The present disclosure relates to a type of superconducting magnet useful as part of an MRI system in a combined MRI and radiation-therapy equipment. An important advantage of a combined MRI and radiation-therapy apparatus is that MRI imaging may be performed near-simultaneously, or in some arrangements, simultaneously, with radiation therapy, to ensure accurate location of treatment targets such as tumors and real-time monitoring of the effectiveness of applied radiation therapy. By combining MRI imaging and radiation therapy, the radiation can be targeted more accurately and the hospital workflow will be more efficient.
Several attempts have been made to provide combined MRI and radiation therapy equipment, but have often resulted in unwieldy apparatus and/or restricted operating parameters such as magnetic field strength or radiation dose.
Two types of radiation may be employed in radiation therapy techniques applicable to the present disclosure:                charged particles; and        electromagnetic radiation.        
Charged particle radiation involves a beam of charged particles, such as electrons or helium nuclei, accelerated by an electrostatic or magnetic accelerator and directed towards a treatment. Electrostatic or magnetic beam focusing and targeting equipment may be used to ensure that the beam reaches the required treatment area, with minimal impact on healthy tissue which does not require treatment. A difficulty with arranging such treatment in conjunction with MRI imaging is that the background magnetic field required for MRI imaging will deflect and disperse the radiation beam unless it is directed precisely parallel to the magnetic field in the region of treatment. This causes difficulty for access of the beam to the treatment region, and difficulty for the beam to reach the treatment region without passing through a substantial part of a patient's body, which is undesirable due to the increased exposure of a patient to the radiation, and the dispersion of the radiation beam and reduction of radiation dose at the treatment site.
Electromagnetic radiation therapy involves directing a beam of high-frequency electromagnetic waves, such as gamma-radiation or “hard” X-rays towards the treatment region. An advantage of electromagnetic radiation therapy is that the treatment beams are not deflected by the magnetic field of the MRI imaging system, but a disadvantage lies in the sometimes large and costly apparatus required to generate the treatment beams.
Some of the known approaches to providing combined MRI and radiation therapy apparatus appear in the following publications:
Radiotherapy Machine Including Magnetic Resonance Imaging System—WO 99/32189—describes an open magnet with radiation beam parallel to MRI magnetic field, in which the radiation beam does not interact with coils or the MRI field.
MRI in guided radiotherapy apparatus with beam heterogeneity compensators—WO 2004/024235 A1—describes use of a charged particle radiation beam perpendicular to the magnetic field, with local correction to reduce the effects of background field on particle trajectory.
System for delivering conformal radiation therapy while simultaneously imaging soft tissue—US 2010/0113911 A1—describes a split-solenoidal MRI magnet with a radiation source on a gantry which rotates about the magnet axis.
Particle Radiation Therapy Equipment Comprising Magnetic Resonance Imaging Means—WO 2006/136865 A1—describes an open magnet with transverse field and a charged particle beam applied transversely, parallel to the magnetic field.
Integrated External Beam Radiotherapy and MRI System—WO 2007/045076—describes a low-field open magnet with integrated coils and linear accelerator radiation source.
Combined Radiation Therapy and Magnetic Resonance Unit—US 2008/0208036—describes a solenoidal magnet with a linear accelerator within the bore of the magnet, delivering a radial radiation beam.
Device for Radiation Therapy Under Image Monitoring—US 2009/0124887 A1—describes a radiation beam perpendicular to a magnetic field, with a rotating patient table and static radiation source.
Radiation Therapy System—WO 2009/155700 A1—describes an open magnet with a charged particle radiation beam directed parallel to the magnetic field.
Image Guided Radiation Therapy—WO 2011/000077 A1—describes a restraining patient table for combined MRI/radiation therapy.
Method and apparatus for shielding a linear accelerator and a magnet resonance imaging device from each other—WO 2011/008969 A1—describes a split solenoid MRI system with linear accelerator radiation source, and RF shielding to reduce interference between the MRI system and the linear accelerator.